In the industry employing electronic wirings, it is known that a tin whisker grows slowly as time elapses, shortly comes into contact with a terminal on a printed wiring substrate, and causes a short-circuit fault. Up to early 2000s, an occurrence of a tin whisker had been suppressed by adding lead to tin. Recently, due to environmental responses of electronic devices, such as Restriction on Hazardous Substances (RoHS), lead-free raw materials including lead-free solders come into use, and a short circuit caused by a whisker is regarded as a problem again.
In an LN optical modulator in which lithium niobate (LiNbO3) is employed in a waveguide substrate, there is another problem caused by tin contained in a gold tin solder. From a solder in a connection portion between a connector and a housing, a sealing connection portion between an optical fiber and the housing, and the like, tin is transported in the vapor phase to a place between electrodes in the LN optical modulator, is deposited and grows between the electrodes, and causes deterioration of bias stability. In addition to the lead-free tendency, the deterioration of bias stability is also caused due to increased electric wiring portions such as a dual polarization-binary phase shift keying (DP-BPSK) modulator and a dual polarization-quadrature phase shift keying (DP-QPSK) modulator, a narrowed space inside the housing, and the like. Moreover, in DP-QPSK for dual wavelength, in addition to the number of electric wirings, the number of connections with respect to optical fibers is also doubled, and particularly this problem has become noticeable.
In addition, in polarization multiplex-type optical modulator modules such as DP-BPSK modulators and DP-QPSK modulators, when an optical waveguide output port of each modulator and an optical fiber are connected to each other, lens coupling is generally carried out instead of butt joining. In modules having a lens coupling structure, in order to prevent an error burst caused when an optical axis is blocked by particles (hereinafter, optical axis blocking), the modules are assembled in clean environments. In addition, in order to prevent aged deterioration of optical transmission properties (increase of an optical loss) caused by materials such as mist and sol which are transported in the vapor phase and adhere or are scorched on an end surface of an optical waveguide, each component is thoroughly cleaned, and a housing structure, in which the inside of the housing is replaced with dry nitrogen and is sealed, is employed. In this specification, a material in a gaseous state or a material which floats in the space and is transported, such as a particle, mist, and sol, will be generically referred to as a “vapor phase transportation material”. Although the expression of “vapor phase transportation material” differs from the original meaning from the technical viewpoint, the term will be defined as described above in this specification.
In addition, recently, the following tendencies (1) to (3) are in progress.                (1) The significantly narrowed internal space of a housing resulted from the downsizing of housing        (2) The increase of materials and members which may become a source of vapor phase transportation materials due to an increase in the number of components resulted from a highly integrated configuration        (3) The increase of light intensity for lengthening a transmission distance and improving the optical signal-to-noise ratio (OSNR)        
Therefore, a chance for a particle to block an optical axis increases dramatically, and an error burst caused by optical axis blocking has become a serious problem. In addition, due to the dramatically increased vapor phase transportation materials, aged deterioration of optical transmission properties (increase of an optical loss) caused by adhering or scorching of the materials has also become noticeable. Moreover, due to the increase of light intensity,
It becomes noticeably that even though a vapor phase transportation material does not approach an end of an optical waveguide, the material is laser trapped by only passing through the optical axis, so that the material is fixed on the optical axis or causes adhering or scorching on an end surface of a waveguide. A laser trap becomes particularly noticeable in a case where the power density of light is 1×105 W/cm2 (@1.55 μm) or higher.
In addition, an LN crystal itself has a strong pyroelectric effect so that the crystal surface is strongly charged due to a temperature change. Therefore, a charged vapor phase transportation material is likely to be attracted to the surface of an LN crystal. In order to ensure the operational stability, a conductive film (a metal, a semiconductor, a resistor, or the like) such as an anti-charge film or a metal film is generally formed in a substrate of an LN optical modulator, except for a surface having an end portion of an optical waveguide (waveguide end portion surface) (for example, refer to Patent Literature 1, 2, and 3). On the other hand, an antireflection film is sometimes formed on the waveguide end portion surface. However, inmost cases, the waveguide end portion surface remains bare and no conductive film is formed on its surface. Therefore, charged particles and the like are likely to be attracted to the waveguide end portion surface. It is a serious problem particularly in a case of being used in an environment in which the temperature environment changes drastically (for example, a radio-on-fiber (RoF) system and a car network (NW)).